EAST ORLEANS — This time, it wasn't just one fin that closed Nauset Beach to swimming.

On Thursday, Orleans officials faced a slew of evidence that great white sharks were close to shore and threatening the thousands of beachgoers enjoying the last great beach days of summer.

All of Orleans' Atlantic Ocean beaches, including its largest revenue generator, Nauset Beach, remained closed to swimming Friday, and town officials announced the ban would last through the busy Labor Day weekend.

Their action came on the heels of Chatham officials banning swimmers at the town's ocean-facing beaches for the rest of the season because of sharks.

On Friday, Karen Krawczyk trudged across hot sand back toward the Nauset parking lot after spending the morning at the beach despite lifeguards allowing only wading to the knees.

"I would love to," she said when asked if she wanted to dive in the water.

A sign posted at the front gate warned beachgoers that there was no swimming because of "marine animals" in the water.

Everyone knew they weren't talking about dolphins.

On Thursday, the choice to close Nauset Beach was relatively easy.

State Division of Marine Fisheries shark researcher Greg Skomal was working with commercial fisherman Bill Chaprales of Cape Cod Sharkhunters looking to put scientific tags on great white sharks off Monomoy in Chatham. A spotter pilot working with other commercial fishermen hailed the boat, notifying them of a shark off Nauset Beach. George Breen, the pilot who works for Skomal and Chaprales, also spotted a great white about two miles south of the main swimming area at Nauset, also headed north.

Skomal's electronic equipment also detected a signal from a tagged shark in the area; it, too, was swimming north.

Skomal tagged two sharks Thursday, including one that came within 30 feet of shore. Close enough, he said, that those on the beach could see it as clearly as he could.

Even though they couldn't go in the water, some people were still paying the $10 to get into Nauset Beach on Friday morning, although only about a third of the parking lot's 900 spaces were taken by noon. Typically, Labor Day is busy through Sunday, said Orleans park and beaches superintendent Paul Fulcher.

On this weekend last year, the town collected parking fees for 750 cars on Saturday, 500 on Sunday and 300 on Monday. That's more than $15,000 in revenue.

Town coffers weren't the only ones being hurt by the lack of beachgoers.

"If they don't come into the parking lot, they don't come to Liam's," said John Ohman, the owner of Liam's at Nauset Beach, a popular take-out restaurant right on the beach.

He had to send some of his help home Friday on what should have been a busy weekend.

When he decided to close Chatham's ocean beaches to swimming, parks director Daniel Tobin cited a preponderance of evidence that sharks were coming closer to shore. He noted data from acoustic tags that Skomal has fixed to at least 10 sharks this year, as well as reports from fishermen and the general public. He also worried about having fewer eyes on the water as the number of lifeguards and boaters declined.

But Skomal said the lack of data prevents him from knowing for sure if great whites are now hunting in the shallow waters away from the major gray seal colony on Monomoy.

"I wish I had 20 years of data. I'd be happy with five solid years of data," Skomal said Friday.

In response to requests from towns, he has added more receivers capable of recording acoustic signals from tagged sharks.

He has also added personnel to download data from the receivers at least once a week to provide more up-to-date information.

There are now four listening posts off Nauset Beach. With a higher number of tagged sharks this year, the receivers are recording more pings. But tagging studies done so far have been inconclusive, showing some sharks heading four miles offshore and others cruising along shore.

"The question is, are sharks using Nauset more or is it just a function of the fact we expanded the number of receivers?" Skomal said.

What is known is that great whites are likely making the trip between the big seal colony on Monomoy and a large group in North Truro near Head of the Meadow Beach.

Skomal believes an increased number of sharks are coming to Cape waters and that competition in the prime hunting grounds off Monomoy might be pushing some to explore and broaden the hunt to areas to the north.

Unfortunately, that is also where the region's most popular beaches are.

The question towns must consider, Skomal said, is how to best protect the public without scaring them away. He liked Fulcher's idea of the town hiring a plane to go up every day over the holiday weekend to check on the local waters before deciding whether to open for swimming.

"It's expensive, but $1,000 a day for three days is probably worth it," he said.

It's time for towns to become more proactive and investigate how other shark-prone areas, such as Australia and South Africa, deal with issues such as detection, warning and rescue.

"Beach managers there have their plans in place," he said. "We're at a stage now where coastal towns on the Cape need to be more forward with adopting similar protocols and strategies."

Skomal hopes to develop a prototype acoustic system that could give beach managers real-time warnings on the location of tagged sharks. He estimates it would cost $50,000 to $100,000 to implement and is looking for private sources of funding.

The Orleans Chamber of Commerce and the town are just now adjusting to the effect sharks might have on businesses, said chamber executive director Mary Corr.

"We don't ever want to do anything that discourages people from coming into town, but, on the other hand, public safety is a major concern," she said Friday.

When nesting piping plovers first started to close off-road access to beaches, the chamber hired a lobbyist to try to make the point in Washington that the town's beaches were too valuable economically to be closed.

"That calmed down, and now we have sharks," Corr said.

On Friday, Andrew McDonald, from Buffalo, N.Y., said he has been coming to Nauset Beach for his summer vacation for seven years. But his two sons chafed at having to stay on shore in the late summer heat Friday, their last day of vacation. And McDonald mused that more frequent shark closures might change his future plans.